Display devices that render image, graphic, and textual information are widespread. Such devices are found in handheld, portable, and fixed-location electronic devices such as mobile smart-phones, laptop computers, computer monitors, and televisions. Such displays typically include an array of light-emitting (or light-reflecting) elements formed on a substrate to represent information controlled by an electronic controller. Color displays include light-emitting elements organized into multi-color pixels. Each multi-color pixel includes multiple, single-color sub-pixels that each emit or reflect a different color of light. A typical pixel in a multi-color emissive display has a red light-emitting sub-pixel, a green light-emitting sub-pixel, and a blue light-emitting sub-pixel. The pixels are usually arranged in a two-dimensional array. The three colors define a full-color gamut for the color display.
Referring to prior-art FIG. 10, a flat-panel color display system 1 includes a controller 41 receiving an image signal 42 that is rendered by the controller 41 into an output display signal 45 for controlling a display 5 formed on a substrate 8. An array of pixels 11, each having a red light-emitting sub-pixel 50, a green light-emitting sub-pixel 52, and a blue light-emitting sub-pixel 54 is formed on the substrate 8. Thin-film transistor circuits 9 control the sub-pixels 50, 52, 54 in response to the display signal 45 from the controller 41. A variety of flat-panel light-emitting color displays 5 are known in the art, for example liquid crystal displays (LCDs), inorganic light-emitting diodes (LEDs), organic light-emitting diode displays (OLEDs), and plasma displays. Reflective displays are also known, for example reflective LCDs, and electro-phoretic displays, as are projected displays.
Display characteristics include brightness, resolution, a high fill factor, and color gamut. The brightness of a light-emitting display is limited in part by the amount of power that is converted to emitted light. The resolution of a light-emitting display is limited by the size of the light-emitting elements on the substrate. The fill factor specifies the percentage of the substrate area that is used to emit or reflect light and can influence the efficiency and life-time of the display. The color gamut is determined by the saturation of the emitted colors. A desirable light-emitting flat-panel display has high brightness, high resolution, high efficiency, a large fill factor, and a large color gamut. For low-resolution displays, a large fill factor is desirable to avoid perceptible dark areas in the display. Therefore, color displays with a large fill factor and small pixels capable of efficiently transforming electrical power into highly saturated colors are desirable.
In order to increase the color gamut of a color display, pixels with more than three colors of light-emitting sub-pixels have been proposed. For example, as shown in FIG. 11, an extended-color-gamut pixel 18 includes a red light-emitting sub-pixel 50, a green light-emitting sub-pixel 52, a blue light-emitting sub-pixel 54, a yellow light-emitting sub-pixel 56, and a cyan light-emitting sub-pixel 58. As illustrated in FIG. 12, U.S. Pat. No. 7,483,095 entitled “Multi-Primary Liquid Crystal Display” discloses a display with pixels that each include eight sub-pixels emitting light of five different colors. Three of the colors are repeated twice. Referring to FIG. 12, the extended-color-gamut pixel 18 includes red light-emitting sub-pixels 50, green light-emitting sub-pixels 52, blue light-emitting sub-pixel 54, yellow light-emitting sub-pixels 56, and cyan light-emitting sub-pixel 58.
Furthermore, because the human vision system perceives luminance signals at a higher spatial resolution than color signals, some color light-emitting sub-pixels can be present at a lower spatial resolution. For example, U.S. Pat. No. 7,495,722 entitled “Multi-Color Liquid Crystal Display” discloses a display with four-color light-emitting pixels emitting red, green, blue, and yellow light alternating with four-color light-emitting pixels emitting cyan, red, green, and blue light, as illustrated in FIG. 13. Referring to FIG. 13, a first extended-color-gamut pixel 18A includes a red light-emitting sub-pixel 50, a green light-emitting sub-pixel 52, a blue light-emitting sub-pixel 54, and a yellow light-emitting sub-pixel 56. A second extended-color-gamut pixel 18B includes a red light-emitting sub-pixel 50, a green light-emitting sub-pixel 52, a blue light-emitting sub-pixel 54, and a cyan light-emitting sub-pixel 58.
Each sub-pixel 50, 52, 54, 56, 58 and associated thin-film transistor circuits 9 (FIG. 10) occupy some portion of the substrate 8. Thus, such extended-color-gamut pixels 18 require a larger substrate area. This increase in area reduces the resolution of the display. Alternatively, the light-emitting area (fill factor) of the sub-pixels is reduced, consequently reducing the lifetime or brightness of the display. (For example the lifetime of OLED materials varies inversely with the emitting area of the materials for a given light output.) The efficiency of the light emitters can also be reduced when the area of a light-emitter is reduced at a given brightness because the power density is increased.
There is a need, therefore, for an improved color display device that improves efficiency, color gamut, and resolution.